A rigid marine sail comprises sail portions (6B) pivotably movable between an open and a closed position about an axis forward of a rotatable mast (1). Signals representative of the wind velocity at predetermined time intervals and signals representative of the wind direction at said intervals are each smoothed (16) and, on the basis of the smoothed signals, a determination is made whether the sail portions are to be open or closed, and the sail is operated accordingly (11,17,18,19). An optimum sail angle is determined for maximum propulsive effect when open and minimum said wind resistance when closed, and the mast and sail is rotated (5,20,21,22) as a whole in accordance with the optimum angle. This permits automatic and effective utilization of the wind force when of a suitable velocity in a suitable direction.